Myth or Fact: DO Depletion by Pumps and Use of Eductors

RupertofOZ

SuperVeg wrote:

What are your theories on why it was discarded ?
Too capital/energy/labour intensive?
too troublesome perhaps ?

All of the above....

And with specific regard to aquaponics.... unworkable... unless you really pre-filter the water to an enth degree....

SuperVeg

yeah ok.
Filtering that much would need plenty of power and/or a really big filter too. Which then needs cleaning. Cleaning filters is probably my least favourite job, I love my compost worms in the GB :notworthy:

mantis

Sounds like crap to me

Journeyman

Not necessarily crap, nbut unlikely. DO is O2 molecules that 'slide' into spaces between the H2O molecules in the liquid. The faster the water molecules move the less likely it is the DO can remain in the holes or gapsd, which is why, as you heat water, you begin to see the DO come out as bubbles.

This might be a reason for DO to be reduced by a pump EXCEPT for the fact the pump is very unlikely to be moving molecules around like that. Pumps deal in far larger quantities of water at a time so the DO will be moved along with the H2O.

Easy way to test would be to look at a submerged pump - if DO is being reduced you SHOULD see bubbles coming from the pump even when you are not aerating.

SuperVeg

Hard to test though. It would be difficult to ensure absolutely ZERO air bubbles are entering the pump in the first place.

RupertofOZ

Journeyman wrote:

Easy way to test would be to look at a submerged pump - if DO is being reduced you SHOULD see bubbles coming from the pump even when you are not aerating.

Well I've never seen bubbles coming from any of my submerged pumps...

Anyone else....

Mythbusted... :lol:

ThommoPerth

PLJ wrote:

:laughing3: Haha. You are funny guy, Charlie - I kill you last!
Some of us actually have work to do outside

, you know, where there are no computers? Not complaining coz I love it, but it was really good to come back inside as the rain started falling and find so many excellent and educated responses to my question.
Thanks to all of you for going to the trouble of drawing on your experience or even pulling out the reference books to help me get my head around this one. On the balance of your responses I would say that this claim has to go in the myth basket.

RupertofOZ wrote:

Do tell... who was this "expert"????

I don't want to name names, R'OZ, but I have his business card on my desk. Apparently he has worked for NASA on a big aeroponics project, which I promptly looked up since I hadn't previously even heard of aeroponics. It is an exciting area of development - have a look:
http://en.wikipedia.org/wiki/Aeroponics#NASA_inflatable_aeroponics
Even if my expert acquaintance is off the mark re the oxygen depletion of pumped water, he certainly knew his stuff, generally. At the time I bumped into him out the back of a warehouse, he was hunched over some conical drums and pipework with a team of gnomes creating a sophisticated looking vortex filter/biofilter setup. That was when I first saw an eductor working its magic and, of course, I have now incorporated one into my big concrete tank system. My expert uses eductors for mass water movement and to compensate for the (real or imagined) loss of oxygen in the systems he designs.

(It has taken quite a while to type this due to having a 6 hour old rejected chick in my hand - long story.)

Ok so what is a eductor ???

PLJ

RupertofOZ wrote:

Whoa... who would have thunk it....

A venturi... to add more oxygen into the water....

I'm surprised that in all the years of this forum.. that no one has ever thought of it... :lol:

As I understand it an eductor is more than a venturi, although its design is based on the venturi principle. Although an eductor will significantly improve aeration if properly installed, I use mine subsurface to dramatically increase the flow created by water returning from my biofilter. Water movement to assist solids removal is more critical to my system than is aeration, at least while the water is cold. It is for this reason that I wanted to be sure about O2 levels in my return water since the water is not exposed to the atmosphere between pump and FT.

I found the following interesting:

A forum search on venturi returns 88 pages of results.
A forum search on eductor returns less than ten instances (excluding mine), most of which were posted by Dave Donley.

PLJ

ThommoPerth wrote:

Ok so what is a eductor ???

Thommo, simply put an eductor is a device which efficiently mixes two fluids/gases such as FT water and air, or just FT water with itself. In an AP setting, its design will typically allow the water passing through it to draw in 4 x its volume of air, or if installed underwater, 4 x its volume in surrounding water. This means that even without a lot of pressure, installing an eductor to a return pipe can greatly increase aeration or water movement in a FT, or both.

RupertofOZ

So how many times does a venturi increase the volume of air, or water... in an AP setting?

PLJ

RupertofOZ wrote:

So how many times does a venturi increase the volume of air, or water... in an AP setting?

I don't claim to know the answer to that question, RupertofOz,

RupertofOZ wrote:

Whoa... who would have thunk it....

A venturi... to add more oxygen into the water....

I'm surprised that in all the years of this forum.. that no one has ever thought of it... :lol:

but "I'm surprised that in all the years of this forum.. that no one has ever thought of"... working it out!

Dave Donley

The way I think of eductors is that you use a little fast moving stream to pull along a much larger slower moving stream. They are curved like they are to make this energy transfer smooth. We often have little fast-moving streams of water coming out our pumps so if all you want is to circulate the water volume then they are appropriate.

PLJ

Dave Donley wrote:

The way I think of eductors is that you use a little fast moving stream to pull along a much larger slower moving stream.

That is spot on, Dave, and it happens to be in the ratio of about 1:4, fast moving stream to slow moving stream (water, air, gas, molten steel, volatile chemicals, pesticide, liquid fertiliser, whatever.)
If the fast moving stream is in the form of incoming water to your FT then you can position just the end of the eductor nozzle below the surface and draw air into the tank at the rate of 4x the flow rate of the water, for instance 1000LPH water + ~4000LPH air. The air and water are well and truly mixed in the process and it is this efficient mixing characteristic that makes eductors so indispensable in many industries.
If moving the FT water around was your aim then, using the same flow rate as above, fully immersing the eductor will have the effect of drawing an additional 4000LPH of FT water through the eductor, thus providing 5x times the water movement that you would otherwise achieve without the eductor. Seems like good value to me.

Dave Donley wrote:

They are curved like they are to make this energy transfer smooth.

My eductors are straight, not curved, so I can't picture the eductors that you are describing.
I will post a picture of a 0.5inch eductor - straight version - first opportunity.

Ronmaggi

Great... Now I need one of those eductor thingys on the bottom of my fishtank to kick up the solids, and I don't even know what they look like, except that they have a special curve, or not.

RupertofOZ

PLJ wrote:

That is spot on, Dave, and it happens to be in the ratio of about 1:4, fast moving stream to slow moving stream (water, air, gas, molten steel, volatile chemicals, pesticide, liquid fertiliser, whatever.)
If the fast moving stream is in the form of incoming water to your FT then you can position just the end of the eductor nozzle below the surface and draw air into the tank at the rate of 4x the flow rate of the water, for instance 1000LPH water + ~4000LPH air. The air and water are well and truly mixed in the process and it is this efficient mixing characteristic that makes eductors so indispensable in many industries.
If moving the FT water around was your aim then, using the same flow rate as above, fully immersing the eductor will have the effect of drawing an additional 4000LPH of FT water through the eductor, thus providing 5x times the water movement that you would otherwise achieve without the eductor.

Serious question PLJ... where did your draw your info from... got a link at all??

Backyard Aquaponics A place to discuss aquaponics

Myth or Fact: DO Depletion by Pumps and Use of Eductors

Backyard Aquaponics
A place to discuss aquaponics